Read any article about or review of any vehicle powered by the 6.4-liter Hemi SRT V-8 engine—for example, our recent look at the 2013 Dodge Challenger SRT8 392—and you’re like to find mention of an “active intake manifold.” And likely there’s no explanation of what that exactly is.
If you don’t know, here’s the answer to the question, “What is an active intake manifold.”
It has to do with optimizing runner length on an intake manifold and the ram effect of a naturally-aspirated kind of supercharging. An engine engineer wants as much as possible to have a positive air pressure on the manifold side of the intake valve so that when the valve opens, the intake charge doesn’t have to overcome inertia, or even low pressure, as the downward motion of the piston draws the charge into the cylinder.
The problem is that at different engine rpm the optimal length of intake runner changes. What’s good for low rpm grunt isn’t the best for horsepower at high revs.
Normally the choice is to compromise and pick one end of the rev range—or somewhere in the middle—depending on he intended use and desired characteristics of the engine. It’s all part of the art and science of choosing valve lift and timing, for much the same reasons as manifold length and generally more important, which is why variable valve timing and even lift are used even on the lower end of the automobile price range. It’s about getting the best bang in terms of both power and fuel economy for the buck.
Active intake manifold tuning is less common than variable valve timing and lift, based on cost and diminishing returns and all that, so it’s often limited to more expensive engines. Manufacturers may call it different things, often with cute marketing department-invented terminology, but Chrysler just calls it active intake manifold.
The active intake manifold in the 6.4L HEMI SRT V8 consists of shaft-mounted butterfly valves—see the illustration—in each cylinder runner of the intake manifold. The valves are on two individual shafts (for each back of the V-8) and flip and closed simultaneously rather than being independently controlled.
Getting techy, in the Chrysler SRT8 engine, the shaft is turned by a pulse width modulation (PWM) device and a gear on the back of the intake manifold within the PWM controller and connects to the shafts.
The default position for the valves is closed, which directs the intake air, or charge, to long runner mode. Long runner mode allows the airflow to tune properly and enter the chambers for the max torque available.
However, at 4800rpm wide open throttle, the active intake manifold valves are opened, giving the intake charge a straight shot into the cylinder, bypassing the long runner. This mode is for obvious reasons referred to as Short Runner. Short Runner mode allows the engine to pack the chamber with as much airflow as possible and therefore make the max horsepower possible.
And that is what “active intake manifold” means.